J. Phys. Chem. 1992, 96, 4220-4225
4220
Addition of 0’ to IO4- In a Crystalline Matrix: A Pentacoordinated Iodine(VII1) Species Studied by ESR and Optical Spectroscopy J. R. Byberg Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark (Received: July 18, 1991; In Final Form: January 28, 1992)
The paramagnetic species IOs” is produced from IO4- embedded in KClO4 crystals by thermal addition at room temperature of radiation-induced 0-. The observed spin Hamiltonian of 10;- is consistent with a pentacoordinated structure having near-equivalent “normal” 1-0 bonds to four oxygens and a weaker bond to the fifth oxygen, which holds half of the electron spin density. The mechanism of formation and the geometry of the host lattice indicate that IOs” is a distorted trigonal bipyramid. Photolysis of IOs2- produces the paramagnetic complex [0;,I03-]. IOs2- has an optical absorption with A, = 530 nm, as have the O-/OH adducts of periodate in aqueous solution reported earlier. Hence the present results are taken to suggest that these adducts, like IO?-,are pentacoordinated iodine(VII1) species. The formation of the adducts is reformulated accordingly.
Introduction Addition of 0-and OH to periodate has been reported to occur in pulse radiolysis and flash photolysis of alkaline and neutral aqueous solutions.’ Regardless of the periodate species present in the solution (HzI063-at p H 1 12, H31062-and IO4- a t 7 I pH I12), the resulting transient adducts exhibit a broad optical absorption band with A, near 525 nm. This does not imply, however, that the same adduct is formed in all cases; rather, the observed kinetics of decay indicates that the band at 525 nm in fact represents several distinct species with similar spectra. The structures of these species are unknown. It has been suggested2 that they might be peroxy compounds in which 0-or OH is bound to one of the ligands of periodate. Thus the formal oxidation state of iodine in these adducts should be VI rather than VIII, as would result from direct attachment of O-/OH to iodine. However, the ease with which periodate ions in aqueous solution switch between tetrahedral and octahedral forms via a pentacoordinated intermediate3 makes a direct attachment of O-/OH as an additional ligand to iodine a plausible alternative to the peroxy model, at least for the tetrahedral periodate species IO4-. With the purpose of elucidating this structural problem by means of a detailed spectroscopic examination of one of the pertinent systems, we have now prepared the 0-adduct of IO, embedded in a single crystal of KC104.
Experimental Section KC104 and KIO, were Merck, analytical grade. Single crystals of KC104:KI04in the form of plates of dimensions 3 X 2 X 1 mm with the large face parallel to (001) were obtained from an aqueous solution of 0.2 mol of KI04/m01 of KC104 by evaporation at room temperature. Crystals containing IO4- labeled with 170 (KC11604:K11704) were obtained from solutions made with water enriched to 20% 170(obtained from Stohler) by slowly cooling the solution from 40 OC to room temperature in a closed vessel. KC104 labeled with I7O, used to prepare crystals of KC11704:KI’604,was made by electrolysis with platinum electrodes of NaCl in water containing 20% ‘’0,addition of the equivalent amount of KCl, and three recrystallizations in cold water to remove NaCl and residual KC103. Irradiations with X-rays were made by means of an OEG-60 Machlett tube with tungsten target, operated at 50 kV and 32 mA. The distance from the beryllium window of the tube to the sample was approximately 4 cm. Bleachings of the irradiated crystals were made with the visible part of the emission from an Osram HQL high-pressure mercury (1) Klaning, U. K.; Sehested, K. J . Chem. Soc., Faraday Trans. I 1978,
.(2) Klaning, U. K.; Sehested, K.; Wolff, T. J . Chem. S O ~Faraday . Trans.
lamp from which the glass envelope had been removed. Sample temperatures in the range 2.7-500 K were obtained with a Heli-Tran liquid helium cryostat (Air Products) and with a Varian V-4547 variable-temperature device. The Varian E-15 ESR spectrometer was operated a t 9.3 and 35 GHz. The static field Bo was monitored by a Bruker ER-035 NMR gauss meter, while the microwave frequency was measured either with a Marconi frequency counter (9.3 GHz) or with an EIP 548 frequency counter (35 GHz). Optical spectra of irradiated crystals of KC104:KI04 spectra were recorded with a Cary 219 spectrophotometer equipped either with a Glan prism polarizer or with a Rehovoth dichroism accessory Model CD-LD-HC, the latter allowing the simultaneous recording of the absorbance for two perpendicular directions of the electric vector E . The analyzing beam was incident on a (001) face of the sample crystals of which a selected area measuring 1 X 2 mm was bounded by a mask made from stiff aluminum foil. The mask was glued with epoxy resin to a silver rod extending from the cold block of the Heli-Tran cryostat, whereas the samples were attached to the mask with adhesive wax rather than glued directly to the tip of the silver rod. This ‘soft” coupling to the cryostat, adopted to reduce the risk of producing additional cracks in the samples during annealings (KC104:KI04proved particularly brittle), introduced an offset of the equilibrium sample temperature with respect to the set temperature of the cold block. Hence the sample temperatures quoted in connection with the optical spectra are estimates based on the observed rates of those processes seen also in the ESR experiments. All measurements on a given sample were made at a fixed reference temperature because the thermal expansion of the cryostat would otherwise displace the optically rather imperfect sample with respect to the analyzing beam. Even so, heating and recooling to the reference temperature sometimes produced small changes in the absorbance a t long wavelengths (of the order 0.001-0.003 in either direction) that could not be correlated with chemical changes in the sample. Such sporadic changes were interpreted as shifts of the baseline. and [O,,IO3-], described The two paramagnetic systems, in this paper both occupy anions sites in the orthorhombic KC104 host crystal, and both conform to the C, symmetry of the site, implying that one principal axis of each term in the spin Hamiltonians 7f8 is perpendicular to the mirror plane (a-c). Accordingly, the parameters of 7fswere derived from a set of selected hyperfine transitions defining the angular variations of the signals in the a-c plane, supplemented with the signal recorded with Boilb, by means of the iterative procedure described earlierS4 The hyperfine tensor A(I7O), representing the additional splitting of the IOS2-signal in the samples containing I7O,was obtained as
74. -2818. ---
I 1981, 77, 1707. ( 3 ) Buist, G . J.; Holloway, C. J. Z . Narurforsch. 1977, 826, 114.
(4) Byberg, J. R.; Jensen, S . J. K.; Muus, L. T.J . Chem. Phys. 1966,46, 131.
0022-3654192f 2096-4220%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4221
Addition of 0-to IO4- in a Crystalline Matrix
TABLE I: Parameters in the Spin Hamiltonians of 102- and [0
